In 3GPP (3rd Generation Partnership Project), 3GPP2 (3rd Generation Partnership Project 2), WiMAX (Worldwide Interoperability for Microwave Access) forum and the like, the standardization of a small radio base station that can be installed in user's home, office and the like is being developed. It is assumed that the small radio base station is installed in home, small office and the like by a user who owns the small radio base station, and connected to a core NW using a broadband line such as an ADSL (Asymmetric Digital Subscriber Line) or optical fiber line. Such a small radio base station is generally called a femto base station, a femtocell base station, or a home base station. Further, the size (cover area) of a cell formed by a small radio base station is significantly smaller than that of a macro cell used hitherto. Therefore, a cell formed by a small radio base station is called a femtocell, a home cell and the like. The 3GPP defines such a small radio base station as Home Node B (HNB) and Home evolved Node B (HeNB) and promotes the standardization. HNB is a small radio base station for UTRAN (UMTS (Universal Mobile Telecommunications System) Terrestrial Radio Access Network), and HeNB is a small radio base station for LTE (Long Term Evolution)/EUTRAN (Evolved UTRAN).
In this specification, such a small radio base station is called “home base station”, undo cell formed by a home base station is called “femtocell”. Note that home base stations for UTRAN and E-UTRAN under study by the 3GPP are called HNB and HeNB, respectively, as are referred to in the 3GPP, or they are collectively called H(e)NB.
When a large number of home base stations are connected to a mobile core network (which is hereinafter referred to as a mobile core NW), and communications of a large number of mobile terminals are performed through the home base stations, the load on the core NW increases. Particularly, it is anticipated that traffic for Internet access by the mobile terminals causes an increase in the load on the core NW. In view of this. the requirements for home base stations defined by the 3GPP include support of “Local IP Access (LIPA) function” by the H(e)NB (see Non Patent Literature 1). LIPA is a function of directly offloading, into user's home network at the home base station, the mobile terminal's traffic to/from the Internet and the user's home network without transferring it to the core NW. LIPA is also called “local breakout”.
FIG. 1 shows a specific example of LIPA in a 3GPP mobile communication system (UMTS (Universal Mobile Telecommunications System)/EPS (Evolved Packet System)). In the example of FIG. 1, a H(e)NB 91 is connected to a subnet 34 of a home NW 3. A subnet means a broadcast domain which is a range that a Layer-2 broadcast reaches. The home NW 3 is connected to an IP (Internet Protocol) access network 4 through a broadband router (BBR) 30.
A H(e)NB-GW 50 is placed at the boundary between a mobile core NW 5 and the IP access NW 4, and relays user data and control data between a data transfer entity and a control entity (SGSN/S-GW 51, GGSN/P-GW 52 etc.) located within the mobile core NW 5 and the H(e)NB 91. When a public network such as the Internet is used as the IP access NW 4, a secure path 41 such as an IPsec tunnel is set up between the H(e)NB-GW 50 and the H(e)NB 91.
When a mobile terminal (UE) 2 connecting to the H(e)NB 91 accesses an external network 6, the H(e)NB 91 transfers transmission data of the mobile terminal 2 to the mobile core NW 5. On the other hand, when the mobile terminal 2 accesses a host 31 located within the home NW or a host 42 within the IP access NW 4, the H(e)NB 91 branches IP packets to the subnet 34 by the LIPA function. For example, the H(e)NB 91 monitors a destination address added to a transmission IP packet of the mobile terminal 2 and, when the destination address corresponds to the host 31, the H(e)NB 91 generates a MAC (Media Access Control) frame addressed to the host 31 and sends it to the subnet 34, without transmitting the transmission packet to a bearer connected to the core NW.
The full names of the abbreviated terms in FIG. 1 are as follows.
SGSN: Serving GPRS (General Packet Radio Service) Support Node
S-GW: Serving Gateway
GGSN: Gateway GPRS Support Node
P-GW: PDN (Packet Data Network)
H(e)NB-GW: H(e)NB Gateway
UE: User Equipment
In Non Patent Literatures 2 and 3, techniques to implement LIPA are proposed. Non Patent Literature 2 proposes a technique that incorporates anchor function of IP communication into the home base station. Non Patent Literature 3 proposes a technique that incorporates NAT (Network Address Translation) function into the home base station.